Decline in offspring viability as a manifestation of aging in Drosophila melanogaster

The evolutionary explanation of senescence proposes that selection against alleles with deleterious effects manifested only late in life is weak becau se most individuals die earlier for extrinsic reasons. This argument also a pplies to alleles whose deleterious effects are nongenetically transmitted from mother to progeny, that is, that affect the performance of progeny pro duced at late ages rather than of the aging individuals themselves. We stud ied the effect of maternal age on offspring viability (egg hatching success and larva-to-adult survival) in two sets of Drosophila melanogaster lines (HAM/LAM and YOUNG/OLD), originating from two long-term selection experimen ts. In each set, some lines (HAM and YOUNG, respectively) have been selecte d for early reproduction, whereas later reproduction was favored in their c ounterparts (LAM and OLD). In the HAM and LAM lines, both egg hatching succ ess and larval viability declined with mother's age and did so with acceler ating rates. The hatching success declined significantly faster with matern al age in HAM than in LAM lines, according to one of two statistical approa ches used. Egg hatching success also declined with maternal age in YOUNG an d OLD lines, with no difference between the selection regimes. However, the relationship between mother's age and offspring larva-to-adult viability d iffered significantly between these two selection regimes: a decline of lar val viability with maternal age occurred in YOUNG lines but not in OLD line s. This suggests that the rate with which offspring viability declines with mother's age responded to selection for early versus late reproduction. We suggest broadening the evolutionary concept of senescence to include intri nsically caused declines in offspring quality with maternal age.